Seismic waves generated artificially have been used for more than 50 years for the imaging of geological layers. In a marine setting, the most widely used waves are reflected waves and, more precisely, reflected compressional waves, recorded by hydrophones and/or accelerometers. In other settings (e.g. land and ocean bottom surveys), shear wave energy can also be of interest. During seismic prospection operations, vibrator equipment (also known as a “source”) generates a seismic signal that propagates in the form of a wave that is reflected from interfaces of geological layers. These waves are received by geophones, hydrophones and/or other types of receivers, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal recorded by recording equipment. Analysis of the arrival times and amplitudes of these waves makes it possible to construct a representation of the geological layers on which the waves are reflected.
A widely used technique for searching for oil or gas, therefore, is the seismic exploration of subsurface geophysical structures. Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine-based seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the seafloor. This profile does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process.
For a marine seismic gathering or acquisition process, as shown in FIG. 1, a data acquisition system 100 includes a ship 102 towing plural streamers 106 that may extend over kilometers behind ship 102. Each of the streamers 106 can include one or more birds 130 that maintains streamer 106 in a known fixed position relative to other streamers 106, and the birds 130 are capable of moving streamer 106 as desired according to bi-directional communications birds 130 can receive from ship 102. One or more source arrays 104a,b may be also towed by ship 102 or another ship (not shown) for generating seismic waves. Source arrays 104a,b can be placed either in front of or behind receivers 140, or both behind and in front of receivers 140. The seismic waves generated by source arrays 104a,b propagate downward, reflect off of, and penetrate the seafloor, wherein the refracted waves eventually are reflected by one or more reflecting structures (not shown in FIG. 1) back to the surface. The reflected seismic waves propagate upwardly and are detected by receivers 140 provided on streamers 106. This process is generally referred to as “shooting” a particular seafloor area, and the seafloor area can be referred to as a “cell,” or geographical area of interest (GAI).
The signals recorded by seismic receivers 140 vary in time, having energy peaks that may correspond to reflectors between layers. In reality, since the sea floor and the air/water are highly reflective, some of the peaks correspond to multiple reflections or spurious reflections that should be eliminated before the geophysical structure can be correctly imaged. Primary waves suffer only one reflection from an interface between layers of the subsurface. Waves other than primary waves are known as multiples, and more strictly, are events that have undergone more than one reflection. Typically, internal multiples, which occur when energy is reflected downward at an interface layer in the subsurface, have a much smaller amplitude than primary reflected waves, because for each reflection, the amplitude decreases proportionally to the product of the reflection coefficients of the different reflectors (usually layers or some sort). Another type of multiple is the so-called “surface multiple” which occurs when a seismic acoustic wave is reflected downwardly from the surface of the water during marine seismic surveying.
Thus, for the foregoing reasons, the acquisition of data in marine-based seismic methods usually produces different results in source strength and signature based on differences in near-surface conditions. Further data processing and interpretation of seismic data therefore typically involves correction of these differences in the early stages of processing. For example, with respect to multiples, Surface-Related Multiples Elimination (SRME) is a technique commonly used to predict a multiples model from received seismic data. Attenuating the surface-related multiples using SRME is based on predicting a multiples model, adapting the multiples model and subtracting the adapted multiples model from the received seismic data.
There are certain problems, however, with accurately modeling shallow water seafloor reflectors as part of the overall seismic data processing, such that it is very difficult to reduce or eliminate the influence of the multiples generated by shallow water seafloor reflectors. In this context, “shallow” can refer to a depth which is less than approximately the distance between a source and the closest receiver to that source, i.e., the smallest offset in the seismic acquisition system. One of the reasons these problems occur is because of move-out issues. As will be appreciated by those skilled in the art, move-out refers to the effect that the distance between a seismic source and a receiver (the offset) has on the arrival time of a reflection in the form of an increase of time as a non-linear function of offset. While in some cases upper reflectors, e.g., reflections from the surface of the water, can be determined and eliminated from the acquired seismic data, attempts to correlate the upper reflectors to shallow water seafloor reflectors using 2D convolution operations sometimes fail because the geometry of acquisition only allows prediction for some given offset length, for example with far offsets of between 200 and 400 meters, but not with shorter offsets, which is typically the case with shallow seafloor conditions that reduce the travel distance for acoustic waves between seismic sources and receivers.
Accordingly, it would be desirable to provide methods and systems for predicting shallow water multiple models in order to improve the image of the geographical area of interest.